1. Field of the Invention
This invention relates to a key telephone system for use in PCM (Pulse Code Modulation) telecommunications, more particularly to a key telephone system using a PCM operating system which is capable of permitting voluntary connections of terminal units including telephone sets and data equipments between a central office line and individual terminal units. Such a key telephone system may include a central office line channel trunk, an extension communication channel trunk, conference communication channel trunk, or public address system trunk.
2. Prior Art
For the transmission and exchange of a large number of communication signals and data signals, there are generally those cases in which time division is used and those in which space division is employed.
In the case where time division is employed, call signals and so forth are converted, as it is generally known, into digital signals in encoded forms, which are either "0" or "1" whereby the signals for many call signals are carried and transmitted on a single transmission line. In this multiplexed time-division transmission of signals, the arrangement of the codes and the timing thereof are predetermined, so that the receiving system separates the multiplexed call signals and so forth and rearranges the sequence of data, thereafter performing the digital/analog (D/A) conversion to make it possible to communicate by the system.
On the contrary, in the space division type speech channel, switching operation is performed by closing, with a cross point switch, the input lines and the output lines in numbers corresponding to the individual terminal units. In this case, the signals are transmitted generally as they are, i.e. in the form of analog signals, without being subjected to any analog/digital (A/D) conversion or to any digital/analog (D/A) conversion.
In time division, the PCM system is employed, so that the call channels can be multiplexed to provide as many channels as desired by increasing the bit rate. Hence, the more the channels are multiplexed, the less the cost per channel will be. However, this system has hitherto presented a problem in that it meant a higher cost in a small-scale system because it requires A/D converters, D/A converters, encoders, decoders, speed converters for its transmission of signals in the form of digital codes.
However, in the space division type speech channel, there is a problem in that the number of the cross point switches in the switchboard increases in proportion to the square of the number of the communication circuits if the number of such circuits increases, even though the system, which transmits the signals as they are, i.e. in the form of analog signals, does not require any A/D converter, D/A converter, encoder, decoder, or speed converter although they are necessary for the PCM transmission and exchanges.
Furthermore, in the space division type speech path, it has hitherto been difficult to deal with digital-coded signals. The reason for this is to be found in the delay time (5-6 ns/m) that occurs in the transmission line, causing a deviation in phase between the transmission timing and the reception timing. Thus, it has not been possible to use this system for highly efficient transmission and reception with simple equipment.
The cost per channel of the existing time division type speech channel operated by the PCM system and that of the space division type speech channels are found to be equal to each other in the proximity of 100 channels. Specifically, the space division type results in a lower cost in a system which has terminal units with 100 or fewer channels whereas the time division type by the PCM system turns out to be lower in cost in a system with more than 100 channels, eventually proving to be more effective.
However, recent progress attained in the LSI (Large Scale Integrated Circuit) technology has brought about a reduction of size and an increase in economic efficiency in the time division type speech channel operated with the PCM system, and these features, coupled with more excellent affinity to data equipment, have produced greater overall superiority in this system as such.